Why use pg_reorg?

pg_reorg is handy when you have a large table which has become bloated (see Show database bloat for a useful bloat-detection query). If you are able to hold an AccessExclusive lock on the table for an extended period, you have it easy: just use CLUSTER or VACUUM FULL. However, if your table is busy being accessed by queries which can't wait hours while a CLUSTER/VACUUM FULL completes, you need a solution which will de-bloat your table and indexes while allowing concurrent reads and writes of the table. pg_reorg allows you to do precisely this. See also depesz's summary.